(1) Field of the Invention
The present invention relates to traffic control between communication nodes in ATM (Asynchronous Transfer Mode) communication, and more particularly to an apparatus and a method for ATM traffic supervisory control wherein an idle cell or an administrative cell is used for the traffic control.
ATM is a specific packet oriented transfer mode using an asynchronous time division multiplexing technique. The multiplexed information flow is organized in fixed size blocks, called cells (ATM cell). An ATM cell consists of an information field and a header. The primary role of the header is to identify cells belonging to the same virtual path and virtual channel (VP/VC) connection. Cells are assigned on demand, depending on the source activity and the available resources. Cell sequence integrity on a virtual channel connection is preserved by the ATM Layer.
(2) Description of the Prior Art
In recent years, B-ISDN (Broadband-Integrated Service Digital Network) has appeared on public switched telephone networks as the next stage of the network. The B-ISDN realizes an even more flexible broadband communication network that makes various services including, e.g., voice communication, data communication, very high speed file transfer, communication between local area networks (LAN), and HDTV (high definition television) services, etc., possible, and therefore uses ATM technology described above that is applicable to multi-mediam communication services.
FIG. 1 schematically shows an example of a configuration between communication nodes in a B-ISDN network. In FIG. 1, a subscriber terminal (TE; terminal equipment) 11 is connected to a communication node 12 in accordance with a user network interface (UNI). A DSU (Digital Service ,Unit) 15 is used to send or receive an ATM cell described above between communication nodes 12 in accordance with a node network interface (NNI). An internal structure of each communication node 12 consists of, e.g., ATM adaptation layer (AAL) 13, ATM switch (ATM-SW) 14 and control unit therefor, etc. The AAL 13 performs segmentation of higher layer information into a size suitable for the information field of an ATM cell and reassembly of the contents of an ATM cell information field into higher layer information between the subscriber terminal 11 and the ATM switch 14.
FIG. 2 shows a basic cell structure of an ATM cell. The ATM cell consists of a five octet header containing information of, e.g., a destination path and channel, etc., and a 48-octet information field containing various information, e.g., voice or data, which is segmented by every 48-bytes, and sent from a terminal 11 shown in FIG. 1. FIG. 3 shows a structure of the header in which coding schemes are adopted according to the network node interface (NNI). In FIG. 3, the first twenty-eight bits are available for routing; 12 bits for a virtual path identifier (VPI) and 16 bits for a virtual channel identifier (VCI). The VPI indicates a communication path and the VCI indicates a channel used in the indicated communication path. A payload type (PT) field (3 bits) indicates a type of information written in the information field. A cell loss priority (CLP) field (1 bit) is used for congestion control, and if the CLP is set (CLP value is 1), the cells are subject to being discarded, depending on network conditions. If the cell is not set (CLP value is 0), the cell has a higher priority. A header error control (HEC) field covers the entire cell header and the code used for this function, that is a CRC (Cyclic Redundancy Check) code, is capable of either single-bit error collection or multiple bit error detection.
FIG. 4 and 5 show examples of the header structure of an ATM cell. The former is the header of an idle cell used to delineate a boundary between adjacent cells, that is a so-called "HEC delineation". The idle cell has specific header values, i.e., VPI/VCI bits and PT bits are all zero, and only a CLP bit is 1 as shown in FIG. 4, and also the HEC bit pattern of the idle cell is fixed at "01010010" as the result of CRC calculation. The HEC delineation described above is performed by detecting the fixed pattern of the HEC field of an idle cell. The latter is the header of an administrative cell. The administrative cell is identified by the value of PT bits "110" as shown in FIG. 5, and used for management of resources.
Again, in FIG. 1, data, which is segmented by the AAL 13, sent from a terminal 11, is applied to a communication path set up by an ATM switch 14. The ATM switch 14 performs high speed ATM switching operation using data (VPI/VCI) of the cell header. However, when detecting the condition of congestion caused by, e.g., an excess of input data over channel capacity, etc., during data transfer, the ATM switch 14 discards partial data from the terminal 11, depending on the path conditions. In such a case, the ATM switch 14 checks the ClP bit of a cell header, and the cell of which the ClP bit is 1 is discarded prior to the cell of which the CLP bit is 0. Usually, the CLP bit of such a voice cell that has a large amount of redundancy and little influence on the cell discard described above is set (CLP value is 1), and conversely the CLP bit of a data cell is not set (CLP value is 0). FIG. 1 illustrates that terminals 11 of both sides combined with respective communication nodes 12 are connected to each other through VP 16 and VC 17.
As described above, in ATM communication the cell discard described above is caused by the condition of congestion. This is very different from conventional STM (Synchronous Transfer Mode) communication in which predetermined data is transferred in a fixedly assigned time-slot so the data discard as described above is never caused. Therefore, in the conventional STM communication there is no need to collect traffic information. Conversely, in the ATM communication there is need to collect traffic information for traffic control so as to manage and keep a network in good condition.
However, in a prior art, a conventional traffic supervisory control apparatus continuously or statistically measures the number of incoming valid information cells and uses the measured cell count as traffic control information (refer to Japanese Unexamined Patent Publication No. 2-272939). In that way, there is a problem that it cannot detect a burst traffic condition that is caused by a concentration of cells incoming or outgoing for a short time, though ATM basically has the character of burst data transmission. Also, in the case of constructing a new network or extending an existent network, it is necessary to set several system parameters for traffic control of the new or existent network, depending the network conditions. However, in such a case there is a problem that it is very difficult to set the parameters described above, particularly to set the most suitable threshold value for starting the operation of the traffic control, and therefore enormous simulation of traffic in the network described above is required in order to precisely decide the parameters.
Further, since cell discard as described above is caused in the ATM communication, it is necessary to conveniently exchange communication paths when the condition of congestion is caused or expected by ordinal monitoring of the paths. In this case, conventionally, each communication node described above sends traffic information thereon together with a network control signal to a network management unit for performing the network management through a so-called control path. However, the traffic information, e.g., the number of cells sent and received for a unit-time, etc., increases in proportion to an increase in the number and so the rate of circuit capacity occupied by the traffic information increases, so that the rate of circuit capacity available to the network control signal decreases, and thereby there is caused a problem that enough circuit capacity for transferring the network control signal to perform the real-time network control cannot be secured. This problem is significant-in that the centralized control over total networks becomes difficult, in particular that for a network in a congested condition, which needs the real-time control most of all.
Furthermore, in a prior art ATM cells having a specific value of VPI/VCI are used as a transmission medium of network control information and network supervisory information. Thus, the cells are easily influenced by an ATM switch unit, and therefore there is a problem that the cell transmission is delayed by fluctuation of ATM switches and a queuing of the cells at the ATM switch unit.